1. Technical Field of the Invention
The present invention relates generally to a monopulse radar apparatus designed to determine the azimuth of a target, and more particularly to an improvement on such a monopulse radar apparatus which is capable of discriminating between targets located close to each other within a radar detectable zone with high accuracy.
2. Background Art
Automotive radar systems are known which are designed to track a target object such as an obstacle or a preceding vehicle for cruise control and/or anti-collision control. It is important for such automotive radar systems to obtain azimuth data for specifying an accurate positional relation between a radar-mounted vehicle and a target as well as the distance to and relative speed of the target. This is accomplished, for example, with beam scan systems or monopulse systems. The beam scan systems, as shown in FIG. 11(a), measure levels of returns of radar waves whose directivities are different from each other to obtain a received signal level distribution, as shown in FIG. 11(b), and selects one of the returns whose signal level is the greatest in the distribution as indicating the azimuth or angular direction of a target object. The monopulse systems, as shown in FIG. 12(a), receive radar returns simultaneously through a pair of receiving antennas a and b spaced slightly from each other (by a distance D in the drawing) to determine a phase difference between the received signals which arises from a difference in distance d (=Dxc2x7sin xcex8) the radar returns have traveled if the angle which the direction of incoming of each of the radar returns makes with a line perpendicular to a front plane of the receiving antennas a and b is defined as xcex8 or an amplitude difference between the received signals (see FIGS. 13(a) and 13(b)) which arises from a difference in beam directivity of the receiving antennas.
It is possible for the monopulse systems to measure the azimuth of the target object accurately only in an area where beams of the two antennas overlap with each other (will be referred to as a monopulse area below). Some of the monopulse systems, thus, increase a detectable range by using more than three receiving antennas arrayed so that adjacent two of the receiving antennas form the monopulse area for measurement of the azimuth. For example, Japanese Patent First Publication Nos. 9-152478 and 62-259077 teach such systems.
Improvement of measurement accuracy of the azimuth in the beam scan systems requires formation of fine beams, which requires increase in size (i.e., an aperture) of the antennas. However, when the beam scan systems are used as an automotive radar, mounted antennas are limited in size, which may lead to a difficulty in obtaining desired accuracy of the measurement.
The monopulse systems have a drawback in that when a plurality of targets are present at the same interval away from a radar-mounted vehicle such as when two automobiles are running side by side in front of the radar-mounted vehicle, it will cause an error to occur in measuring the azimuth. Specifically, when two automobiles are running in parallel to each other at substantially the same speed, radar returns from the two automobiles which have substantially the same frequency are received by the monopulse system as a composite wave. Usually, automotive radar use millimeter waves. The wavelength of a radar return will, thus, be on the order of several mm, so that the phase of the radar return changes greatly even when the distance to the target is changed in units of millimeter.
When two target automobiles are running side by side, but only one of them is located within a monopulse area (at a position, as indicated by {circle around (1)} in a graph of FIG. 14(a)), a radar wave (i.e., a vector, as indicated by a broken line {circle around (1)}) reflected from the one of the target automobiles in the monopulse area and a radar wave (i.e., a vector, as indicated by a solid line {circle around (2)}) reflected from the other target automotible lying out of the monopulse area (at a position, as indicated by {circle around (2)} in the graph) are different in signal level when received by the monopulse system, so that a composite wave (i.e., a vector, as indicated by a solid line) in which the two reflected radar waves are mixed will approximate to the radar wave reflected from the target automobile within the monopulse area, thereby enabling information on the azimuth to be obtained accurately. However, when two target automobiles are, as shown in FIG. 14(b), both located within the monopulse area, radar waves (vectors, as indicated by broken lines {circle around (1)} and {circle around (2)}) reflected from the two automobiles, received by the monopulse system have substantially the same signal level, so that a composite wave thereof (i.e., a vector, as indicated by a solid line) shows the direction greatly different from angular directions of the target automobiles. This also causes only one of the target automobiles to be detected.
Specifically, in the monopulse system, radar waves reflected from a pair of targets lying within the same monopulse area are mixed in vector to produce a composite wave which is different in phase and amplitude from either of the reflected radar waves, thereby making it difficult to measure the azimuth of the targets using the phase and amplitude of the reflected radar waves.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a radar apparatus designed to discriminate between targets close to each other for measuring angular directions thereof with high accuracy.
According to one aspect of the invention, there is provided a radar apparatus which comprises: (a) a transmitter transmitting a radar wave; (b) a signal receiver providing antenna beams which overlap with each other to define a plurality of monopulse areas, the signal receiver receiving a return of the radar wave from a target object in each of the monopulse areas to produce a pair of input signals; (c) an angular direction data determining circuit processing the input signals produced in each of the monopulse areas to obtain angular direction data, in time sequence, each indicating an angular direction of the target object based on differences in one of amplitude and phase between components of the input signals; and (d) a variation determining circuit determining a variation in angular direction data obtained, in time sequence, in each of the monopulse areas. If the variation is within a preselected allowable range, the variation determining circuit determines its angular direction data as being effective in determining an angular direction of the target object correctly.
For instance, when two target vehicles are running side by side in front of a radar-mounted vehicle, the radar apparatus receives a mixture of returns of a radar wave from the target vehicles in the monopulse area. This mixed radar return is different in phase and amplitude from each of the returns from the target vehicles and changes greatly with a slight change in distance between the radar-mounted vehicle and the target vehicles, thus resulting in a great change in azimuth measured by monitoring the mixed radar return cyclically. The radar apparatus of this invention, thus, monitors, in time sequence, the azimuth data in each of the monopulse areas and ignore some of the azimuth data whose time-sequential variation is out of an allowable range when determining the angular direction of each target, which improves stability and reliability of control using the azimuth data.
In the preferred mode of the invention, the signal receiver is designed to provide the antenna beams so that adjacent two of the monopulse areas partially overlap with each other. It is also advisable that the monopulse areas overlap with each other so that the returns from two target vehicles running side by side at the same interval away from the radar-mounted vehicle may be received with different signal levels whose difference is greater than a preselected reference value.
The signal receiver includes a three or more receiving antennas which are located so as to have the antenna beams oriented in different directions, respectively, and so that adjacent two of the antenna beams define one of the monopulse areas.
The signal receiver may include a plurality of receiving antennas arrayed in line to have antenna beams thereof oriented in the same direction and a signal processing circuit summing outputs from the receiving antennas with given weighting to form the beams. In this case, the signal processing circuit may be made to have a so-called phased array antenna structure which includes a phase shifter changing the phase of the antenna outputs to weight the antenna outputs and an adder adding outputs of the phase shifter together or includes an analog-to-digital converter sampling the outputs of the receiving antennas to produce digital signals and an arithmetic circuit performing a complex Fourier Transform on the digital signals in space series along an array of the receiving antennas, thereby forming the so-called digital beams.
The arithmetic circuit may add null dummy signals to the digital signals produced by the analog-to-digital converter to increase the number of signals to undergo the complex Fourier Transform simultaneously more than the number of the outputs from the receiving antennas. This technique is called zero-padding which is taught in, for example, chapter 11 Irregular Vibration and Spectrum Analysis, published by Ohm Company. Specifically, the addition of the dummy signals causes the number of receiving antennas to be increased logically, which will increase the number of antenna beams within the radar detectable zone, thereby resulting in increased accuracy in measuring the azimuth of the target.
If a plurality of targets are within the radar detectable zone at substantially the same distance from the radar apparatus, the formation of the monopulse areas of which adjacent two monopulse areas partially overlap with each other enables effective azimuth data on the same target to be obtained in some of the monopulse areas. For example, assuming that adjacent two of nine monopulse areas M1 to M9, as shown in FIG. 15(a), overlap with each other in three-fourths (xc2xe) thereof, and that two targets T1 and T2 are present, the monopulse areas M2 to M4 detect only the target T1. The monopulse areas M5 and M6 detect both the targets T1 and T2, but it is impossible to measure the azimuth thereof accurately because returns of a radar wave from the targets T1 and T2 are received with substantially the same signal levels. The monopulse areas M7 to M8 detect only the target T2. The monopulse areas M1 and M9 detect no targets. Specifically, the azimuth data on either of the targets T1 and T2 is obtained two or more of the monopulse areas M1 to M9.
Assuming that adjacent two of seven monopulse areas M1 to M7, as shown in FIG. 15(b), overlap with each other in one-half (xc2xd) thereof, and that the width of targets T1 and T2 is greater than half of the width of each of the monopulse areas M1 to M7, the monopulse areas M2 and M3 detect only the target T1. The monopulse area M4 detects both the targets T1 and I2, but it is impossible to measure the azimuth thereof accurately because returns of a radar wave from the targets T1 and T2 are received with substantially the same signal levels. The monopulse areas M5 and M6 detect only the target T2. The monopulse areas M1 and M7 detect no targets. Specifically, the azimuth data on either of the targets T1 and T2 is obtained two or more of the monopulse areas M1 to M7.
In the above cases, the radar apparatus may include: (a) a transmitter transmitting a radar wave; (b) a signal receiver providing antenna beams which overlap with each other to define a plurality of monopulse areas, the signal receiver receiving a return of the radar wave from a target object in each of the monopulse areas to produce a pair of input signals; (c) an angular direction data determining circuit processing the input signals produced in each of the monopulse areas to obtain angular direction data each indicating an angular direction of the target object based on differences in one of amplitude and phase between components of the input signals; and (d) a grouping circuit, if some of the angular direction data are close to each other within a given range, forming a group including the angular direction data close to each other within the given range; and (e) a determining circuit determining the angular direction data in the group as values effective in determining an angular direction of the target object.
The radar apparatus may alternatively include: (a) a transmitter transmitting a radar wave; (b) a signal receiver providing antenna beams which overlap with each other to define a plurality of monopulse areas, the signal receiver receiving a return of the radar wave from a target object in each of the monopulse areas to produce a pair of input signals; (c) an angular direction data determining circuit processing the input signals produced in each of the monopulse areas to obtain angular direction data each indicating an angular direction of the target object based on differences in one of amplitude and phase between components of the input signals; and (d) a grouping circuit forming groups each including some of the angular direction data which are close to each other within a given range; and (e) a determining circuit determining the angular direction data belonging to one of the groups whose time-sequential variation is within a preselected range as values effective in determining an angular direction of the target object.